1. Field of the Invention
The present invention relates to a fixing device that fixes an image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Heretofore, in an image forming apparatus such as a copying machine, a printer, a facsimile, and an MFP device of these devices, a fixing device is provided to fix a toner image held on a recording medium such as a paper sheet. Generally, the fixing device includes a fixing member heated by a heating source such as a heater and an opposing member that contacts the fixing member to form a nip portion. When an image forming operation is started in an image forming apparatus and a toner image is transferred to a paper sheet, the paper sheet passes through a nip portion between the fixing member heated at a predetermined temperature and the opposing member, and a toner held on the paper sheet is molten to fix an image.
Moreover, in the fixing device, since the paper sheet passing through the nip portion absorbs the heat of the fixing member, the fixing member is controlled to be kept an appropriate temperature using a temperature sensor or the like. In a non-paper feeding region of the fixing member where the fixing member is not in contact with the paper sheet in the width direction thereof during passage of the paper sheet through the nip portion, the heat of the fixing member does not tend to be absorbed. Thus, particularly, in a case where the paper sheets are continuously fed, a problem arises in that the temperature of the fixing member is excessively increased in the non-paper feeding region.
Therefore, heretofore, in order to solve the problem, a fixing device has been proposed in which a shielding member is provided to block heat from a heating source in the non-paper feeding region of a fixing member (see Japanese Patent No. 4130898, Japanese Patent Application Laid-open No. 2008-058833, and Japanese Patent Application Laid-open No. 2008-139779).
However, in the configuration in which the shielding member blocks heat from the heating source, since the shielding member itself is heated by the heating source, it can also be considered that the shielding member is deformed by heat depending on the use situations, for example. In the worst-case scenario, in a case where the shielding member is deformed, it is likely that the function of the shielding member is degraded or a deformed portion interferes with the other members. Thus, some configurations are necessary to suppress these events.
Furthermore, in the fixing devices, the shielding member is configured to be movable. The shielding member is disposed at an appropriate position according to the paper sheet size, so that heat can be blocked in a necessary range, and a heating region corresponding to the paper sheet width can be secured.
The fixing device described in Japanese Patent Application Laid-open No. 2006-71960 uses an induction heating method in which the fixing member is heated by generating a magnetic flux. Here, a magnetic flux shielding member that blocks a magnetic flux is made movable according to the paper sheet size, so that the heating region corresponding to the paper sheet width can be secured.
As described above, in the configuration in which the shielding member is movable, it can also be considered that the shielding member is temporarily returned at an initial position after finishing a printing operation (a fixing process) in order to control the position of the shielding member. However, in a case where the image forming apparatus is stopped in the midway point of the operation due to an abnormality, or in a case where the fixing device is detached or attached, since it is likely that the shielding member is not returned at the initial position, it is necessary to perform the operation of returning the shielding member at the initial position in performing the starting up operation of the image forming apparatus, for example. However, when it takes time to return the shielding member at the initial position in the operation, such a problem arises in that a user or the like has to wait for a long time because the printing operation (the fixing process) is not performed during returning the shielding member at the initial position.
Moreover, no specific structure is disclosed in any of Japanese Patent No. 4130898, Japanese Patent Application Laid-open No. 2008-058833, and Japanese Patent Application Laid-open No. 2008-139779 in which the shielding member is rotatably supported. Depending on the configuration of a support structure for the shielding member, the structures around the shielding member are complicated or increased in size, and it is likely to degrade the flexibility of the layout in the design of the fixing device as well as the image forming apparatus.
Therefore, there is a need for a fixing device that is capable of suppressing the heating of a shielding member, and an image forming apparatus including the fixing device.
Moreover, there is a need for a fixing device that is capable of shortening time to return a shielding member at an initial position, and an image forming apparatus including the fixing device.
Furthermore, there is a need for a fixing device that is capable of rotatably supporting a shielding member using a compact, simple mechanism, and an image forming apparatus including the fixing device.